Mounting large structures such as wind turbine generators in offshore locations involves a number of complexities. Because of the size of the structures they are inherently difficult to transport and there are significant handling issues in achieving a correct alignment of the structure with its offshore support base, jacket or foundation. These difficulties are aggravated when the sea conditions are less than ideal. In relatively shallow waters it is possible to use so-called jack-up barges which, when the jacks are extended to at least partially support the barge on the seabed, provide a stable platform from which the offshore structure can be mounted on its offshore support base. However, it is increasingly required to mount structures such as wind turbines in deeper water where jack-up barges cannot operate. The present invention seeks to provide a method and apparatus which is particularly suitable for the mounting of offshore structures in deeper waters and further which is especially suitable for mounting tall structures or relatively small width, such as wind turbine generators.
More especially, the present invention seeks to provide a system or construction in the form of co-operating apparatus, and a method, by which an offshore structure such as a wind turbine generator can be fully assembled on shore, lifted onto a transporting vessel such as a floating crane barge and moved on the vessel to its final installation position whereat the offshore structure is transferred from the vessel and mounted on a pre-installed foundation or jacket. The system of the invention transports the offshore structure in an upright configuration. In the case of a WTG (and similarly for other tall, narrow offshore structures which typically include a shaft, mast or tower which is arranged vertically in use), the WTG is relatively delicate and is also stiff in the longitudinal (vertical) direction. This makes it especially important to avoid shocks when mounting the WTG on its offshore foundation. Usually, the centre of gravity of structures such as WTGs is at a height which is more than twice the width of the base of the structure so that in handling the structure in an upright configuration, instability of the structure is a major concern.